Project Summary Cardiovascular disease is the leading cause of death and disability in the United States. Mitochondria are vital organelles for heart function because they sustain contractility by providing the heart with energy in the form of ATP through oxidative phosphorylation. Because of this, there is a strong correlation between mitochondrial dysfunction and the development of heart failure. However, the mechanism(s) of origin and precise role in disease progression are not fully understood. The anti-apoptotic BCL-2 family protein Myeloid Cell Leukemia-1 (MCL-1) is widely recognized as an important factor in programmed cell death inhibition. Interestingly, it has also been shown to be essential for maintaining mitochondrial integrity and cardiac function in the adult heart. MCL- 1 exists in two forms, one found in the outer mitochondrial membrane (MCL-1OM) and the other in the mitochondrial matrix (MCL-1Matrix). While studies have implicated MCL-1OM in regulating apoptosis, very little is known about the functional role of MCL-1Matrix. My preliminary data indicate that MCL-1Matrix can regulate mitochondrial morphology by promoting fusion of mitochondria. Furthermore, MCL-1Matrix protects mitochondria from degradation by autophagosomes in response to stress. However, the mechanism by MCL-1 exerts these functions and their functional roles in the myocardium are still unclear. The proposed project will examine the hypothesis that MCL-1Matrix promotes fusion of mitochondria to preserve bioenergetic capacity and protect them from mitophagy and necrosis during energy limiting conditions. This hypothesis will be tested through two specific aims. The first aim will delineate how MCL-1Matrix regulates mitochondrial morphology and cell survival. The second aim will evaluate the functional role of MCL-1Matrix in regulating mitochondrial morphology and function in vivo. This aim will utilize WT and cardiac specific MCL-1Matrix transgenic mice to investigate how overexpression of MCL-1Matrix affects mitochondrial structure/function and responsiveness to fasting and myocardial infarction. These studies will provide important new insights into the relationship between mitochondrial dynamics, turnover and survival in the heart. A better understanding of how mitochondrial function is regulated in the heart under normal and disease conditions such as myocardial infarct will contribute towards future clinical management of heart disease.